A magnetoresistive random access memory (MRAM), which is one of non-volatile memory devices, includes a plurality of magnetic memory cells. It is known that the magnetoresistive effect appears in multi-layer films that are alternately stacked by magnetic layers and non-magnetic layers. Magnetic resistance over a magnetic memory cell indicates minimum and maximum values when magnetization vectors in magnetic layers point in the same or opposite directions, respectively. The same and opposite directions of magnetization vectors in two magnetic layers are called "Parallel" and "Anti-parallel" states, respectively. When magnetic material is employed for a memory device, Parallel and Anti-parallel directions, for example, are logically defined as "0" and "1" states, respectively. The magnetization vectors in magnetic layers are very quickly switched to another direction by an application of a magnetic field over a switching point, and maintain the magnetization direction even without a magnetic field.
The MRAM device normally has magnetic memory cells arranged on intersections of metal lines, which are placed in rows and columns. The MRAM circuit, for instance, is described in a co-pending U.S. patent application Ser. No. 09/128,020 entitled "MAGNETIC RANDOM ACCESS MEMORY ARRAY DIVIDED INTO A PLURALITY OF MEMORY BANKS," filed Aug. 3, 1998 assigned to the same assignee. This patent teaches an MRAM bank that has bit lines, a reference line, and digit lines. The reference line is placed parallel with bit lines and perpendicular to digit lines. On each intersection of these lines, memory cells and reference cells are arranged in rows and columns. The reference cell is employed to sense states stored in the memory cell.
The MRAM device generally is formed on a substrate such as a semiconductor or glass. Magnetic and non-magnetic layers are sequentially deposited on the substrate and etched to form magnetic memory cells, which are 0.2 .mu.m by 0.3 .mu.m in size, for instance. The magnetic layer usually has a 10-100 .ANG. thickness while the non-magnetic layer is 10-30 .ANG. thick. The MRAM device includes magnetic memory cells for storing specific information, and reference magnetic memory cells where a reference state is maintained. In order to read states stored in a magnetic memory cell (active or target cell), a sense current and a reference current are applied to the target cell and a reference magnetic memory cell (reference cell), respectively. The target and reference cells generate voltage drops across the cells that correspond to magnetic resistance values of the target and reference cells, respectively. These voltages are compared to each other to determine states in the target cell. Therefore, it is desirable to provide an MRAM device that has magnetic memory cells without relative variations of the magnetic resistance to correctly compare a target cell to a reference cell.
Accordingly, it is a purpose of the present invention to provide an improved MRAM device that has a high-speed, high-density, and low power memory.
It is another purpose of the present invention to provide an improved MRAM device that has a high reliability reading process.
It is still another purpose of the present invention to provide an improved MRAM device that reduces variations of the magnetic resistance in magnetic memory cells.